1. Field of the Invention
The present invention relates to a high density multilayer printed circuit board. More particularly, it relates to a high density multilayer printed circuit board providing a number of signal layers, electric source layers, and ground layers.
2. Description of the Related Art
The high speed of electronic computers has made it desirable that a printed circuit board for mounting LSI devices be produced at a high density and in a multilayer structure.
Namely, although a small number of signal layers can be operated in a printed circuit board, it is not possible to operate more than ten signal layers. This is because an increase in the number of signal layers inevitably causes a corresponding increase in the numbers of electric source layers and ground layers, giving the resultant printed circuit board an undesirably large thickness.
Conventionally, when a large number of electric source and other layers is formed as a thick board, the lands of the electric source and other layers are electrically connected to a through hole. This through hole must be formed by a drill, and necessitates the drilling of a hole through a correspondingly large number of copper lands; this imposes a heavy load on the cutting surfaces of the drill bit, with the result that these cutting surfaces of the drill bit quickly become notched and blunted.